The invention relates to a powered hand tool for machining and, more particularly, to a powered hand tool for machining magnetizable materials.
The invention is based on a powered hand tool having an enclosed ventilated drive motor, particularly an electric drive motor, in which air passes from the ambient atmosphere into the interior of the powered hand tool to cool the motor during operation. Such hand tools are well known, e.g. the angle grinder. To cool the windings of the electric motors of these powered hand tools, ambient air is as a rule blown through cooling-air ducts passing through the stator winding, a fan wheel mounted on the rotor shaft providing the necessary forced convection.
In enclosed-ventilated motors, the solid particles, occurring in particular in grinding machines, in the cooling-air flow create problems. These solid particles can cause damage on their way through the interior of the motor. Thus, for example, EP-A-0,261,306 teaches a protective means for the end windings of the armature of the electric motor having ribs which project above the end windings and deflect away the stray solid particles, before they can strike the end windings.
However, with the device described there, metal particles drawn in with the drawn-in air cannot be prevented from penetrating into the interior of an electric motor and settling there.
Experience shows that the metal particles settle at the locations where turbulence occurs in the flow of the drawn-in air on account of cross-sectional changes in the cooling-air channel and where there are at the same time powerful magnetic forces. The magnetic flux in the area of the annular gap between the rotor and the stator of an electric machine is particularly strong. It has been found in tests that the metal particles carried along with the drawn-in air mostly collect in this annular gap. The force of attraction of the magnetic field prevailing in the annular gap is so great there that even improved seals or a narrower gap between rotor and stator cannot reliably prevent dust laden with magnetisable particles from penetrating into the annular gap between rotor and stator.
If sufficient steel/stone dust has collected in the annular gap between rotor and stator, failure of the machine occurs through seizure of the rotor on the stator or through clogging of the bearing.